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As an air gun charging cylinder with that "surface only" valve fitted you get 5 years use before hydro (UK and EU use) For diving conversion you add a diving DIN valve £30 and another £30 for the hydro. you get same 5 year hydro but have to add additional periodic visual inspections for submerged use. Hence It's worth nothing for diving. As you dont say where you are from and as it is a UK/EU cylinder set for air gun filling I have assumed your in the EU/UK
Scubaboard is an American forum you will get an American centric reply hence the DOT Dept of Transportation advise and my question regarding your location.
I should add if you are in USA and in addition to the earlier advise that the American DOT federal requirement would be required, As the cylinder was built for a European standard with a M25x 2 thread form metric. The typical Amercan thread form is 3/4 NPSM they dont mix and should never be mixed up.
how is it worth nothing for diving in the EU? a new valve and hydro is still cheaper than buying a new cylinder